Turn on Methods of SCR Adil Sarwar Gate characteristics Gate circuit Gate characteristic of thyristor or SCR • Gate characteristic of thyristor or SCR gives us a brief idea to operate it within a safe region of applied gate voltage and current. • So this is a very important characteristic regarding thyristor. • At the time of manufacturing each SCR or thyristor is specified with the maximum gate voltage limit (Vg-max), gate current limit (Ig-max) and maximum average gate power dissipation limit (Pgav). • These limits should not be exceeded to protect the SCR from damage and there is also a specified minimum voltage (Vg-min) and minimum current (Ig-min) for proper operation of thyristor. • Curve 1 represents the lowest voltage values that must be applied to turn on the SCR and curve 2 represents the highest values of the voltage that can safely applied. So from the figure we can see the safety operated area of SCR is bcdefghb. Introduction • SCR has two stable states IN FIRST QUADRANT Forward blocking and Forward conduction state. • Switching the SCR from forward blocking state (OFF- state) to forward conduction state (ONstate) is known as turning ON process of SCR. • It is also called as triggering. Intro… • With a voltage applied to the SCR, if the anode is made positive with respect to the cathode, the SCR becomes forward biased. • The SCR can be made to conduct or switching into conduction mode BY FOLLOWING METHODS…… Turn on methods • • • • • Forward voltage triggering Temperature triggering dv/dt triggering Light triggering Gate triggering Forward Voltage Triggering • By increasing the forward anode to cathode voltage, the depletion layer width is also increased at junction J2. • It causes increase in minority charge carriers. • This further leads to an avalanche breakdown of the junction J2 at a forward breakover voltage VBO. Forward Voltage Triggering • In practice this method is not employed because it needs a very large anode to cathode voltage. • Once the voltage is more than the VBO, it generates very high currents which may cause damage to the SCR. • Therefore, in most of the cases this type of triggering is avoided. Temperature Triggering • The reverse leakage current depends on the temperature. • If the temperature is increased to a certain value, the number of hole-pairs also increases. • This causes to increase the leakage current and further it increases the current gains of the SCR. • This starts the regenerative action inside the SCR since the (α1 + α2) value approaches to unity (as the current gains increases). • By increasing the temperature at junction J2 causes the breakdown of the junction and hence it conducts. Temperature Triggering • By increasing the temperature at junction J2 causes the breakdown of the junction and hence it conducts. • This type of triggering is practically not employed because it causes the thermal runaway and hence the device or SCR may be damaged. dv/dt Triggering • In forward blocking state junctions J1 and J3 are forward biased and J2 is reverse biased. • So the junction J2 behaves as a capacitor (of two conducting plates J1 and J3 with a dielectric J2) due to the space charges in the depletion region. The charging current of the capacitor is given as I = C dv/ dt dv/dt is the rate of change of applied voltage and C is the junction capacitance. dv/dt Triggering • From the above equation, if the rate of change of the applied voltage is large that leads to increase the charging current which is enough to increase the value of alpha. • So the SCR becomes turned ON without a gate signal. • However, this method is also practically avoided because it is a false turn ON process and also this can produce very high voltage spikes across the SCR so there will be considerable damage to it. Light Triggering • An SCR turned ON by light radiation is also called as Light Activated SCR (LASCR). • This type of triggering is employed for phase controlled converters in HVDC transmission systems. • In this method, light rays with appropriate wavelength and intensity are allowed to strike the junction J2. Light Triggering • These types of SCRs are consisting a niche in the inner p-layer. • Therefore, when the light struck on this niche, electron-hole pairs are generated at the junction J2 which provides additional charge carriers at the junction leads to turn ON the SCR. Gate Triggering • This is most common and efficient method to turn ON the SCR. • When the SCR is forward biased, a sufficient voltage at the gate terminal injects some electrons into the junction J2. • This result to increase reverse leakage current and hence the breakdown of junction J2 even at the voltage lower than the VBO. Gate Triggering • In gate triggering method, a positive voltage applied between the gate and the cathode terminals. • We can use three types of gate signals to turn On the SCR. DC signal, AC signal and pulse signal DC Gate Triggering • In this triggering, a sufficient DC voltage is applied between the gate and cathode terminals in such a way that the gate is made positive with respect to the cathode. • The gate current drives the SCR into conduction mode. • In this, a continuous gate signal is applied at the gate and hence causes the internal power dissipation (or more power loss). AC Triggering • This is the most commonly used method for AC applications where the SCR is employed for such applications as a switching device. • With the proper isolation between the power and control circuit, the SCR is triggered by the phase-shift AC voltage derived from the main supply. • The firing angle is controlled by changing the phase angle of the gate signal. Pulse Triggering • The most popular method of triggering the SCR is the pulse triggering. • In this method, gate is supplied with single pulse or a train of pulses. • The main advantage of this method is that gate drive is discontinuous or doesn’t need continuous pulses to turn the SCR and hence gate losses are reduced in greater amount by applying single or periodically appearing pulses. • For isolating the gate drive from the main supply, a pulse transformer is used.
